 (define (or-gate a1 a2 output) 
   (let ((c1 (make-wire)) 
         (c2 (make-wire)) 
         (c3 (make-wire))) 
     (inverter a1 c1) 
     (inverter a2 c2) 
     (and-gate c1 c2 c3) 
     (inverter c3 output)))
